Nereus, also known as "the old man of the sea", was the son of Pontus and Gaea, and father of the water nymphs Nereids and Nerites. One of the Nereids was Thetis, who was the mother of the famous heroAchilles. He was considered to be a gentle and trustworthy god, who never lied. Gradually, Nereus was replaced by another sea god, Triton.